CODEWITHSUNDEEP
python
huggie
huggie

Reputation: 18237

Python global in module

I'm casually reading Python module hashlib. From the doc I see I can use hashlib.md5() to call up md5. I wonder how that code works? In hashlib.py source code, it appears to me __get_builtin_constructor probably gets called with md5, but I don't know how the code works. Yes I can see md5 is defined for __all__ but how does it get there?

Upvotes: 0

Views: 127

Answers (1)

sberry
sberry

Reputation: 132018

The key is here:

   135 for __func_name in __always_supported:
   136     # try them all, some may not work due to the OpenSSL
   137     # version not supporting that algorithm.
   138     try:
   139         globals()[__func_name] = __get_hash(__func_name)

The __get_hash function maps to either __get_builtin_constructor or __get_openssl_constructor depending on the availability of openssl.

Let's assume openssl is not available. This is how it would look...

The for __func_name line would iterate the keys of

__always_supported = ('md5', 'sha1', 'sha224', 'sha256', 'sha384', 'sha512')

And would add a reference to each in the module scope (globals()). When you call hashlib.md5(), first it uses 

66 def __get_builtin_constructor(name):

To define a callable which is returned. In the case of md5 it returns 

73             return _md5.new

Since it is a callable that is returned, the call is then made because of the parenthesis after hashlib.md5.

Upvotes: 3

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